"I think they're very skeptical about (her story)," Miller said on "CBS This Morning." "But again, 'skeptical' isn't proof and that's why they're trying to work through that and I think that's why her lawyers are slowing that down."

CBS News has also learned later Friday afternoon that Russell was seen leaving in a minivan from her parents' home in Rhode Island and that that FBI agents were parked outside of the home monitoring her movements.

"They will likely keep surveillance logs of where she goes and what she does," said Miller, "but this is not covert surveillance as you might conduct during an investigation. This is what is jokingly referred to a "bumper-lock" surveillance. The subject knows you are there. The real purpose of this is to be able to keep tabs on Russell until they determine whether or not she is a suspect in the bombing conspiracy or not. If information turned up implicating her, and they went to find her and she was gone, that would be a bad thing. So they are covering their flank by watching her until they have a clear determination if she had any role or knowledge in her husbands' plot."

Investigators are looking at a phone call between Russell and Tamerlan Tsarnaev just hours after the FBI released a picture of him. Miller said the "nature of that call" is of particular interest.

"Did (Tamerlan and Dzhokhar Tsarnaev) disclose to her that they were the bombers? Did they say they were going on the run? Did they give her instructions to assist with any escape?" Miller said. "But there's no known tape of that call, so really the account they're going to have to get of that since Tamerlan is dead is hers."

What continues to puzzle the FBI about Russell is what she knew and when, if anything, Miller said. "What they really want to know from her is, you know, here's a woman who dated Tamerlan, who converted to Islam after they were married, who ... lived in the same apartment -- not a big apartment -- where bombs were being constructed and pressure cookers were being bought two or three at a time," Miller said. "They want to know, 'Is it possible all of this was going on concealed from you or did you know it the whole time?'"

Cooperative with police since the initial stages of the investigation, Russell has begun to communicate with law enforcement through lawyers.

Miller explained, "The other day they showed up at her family's place where she's staying now in Rhode Island with a court order to take DNA samples, hair samples and so on to compare with what they found on the bomb.

"From that point on what's being put out is that she's no longer cooperating," Miller said. "I think what we're actually seeing is lawyers doing their job, which is she retained counsel and it was very clear she was developing into a potential target of this investigation and they basically said, 'If you want anything -- it's not that we're not cooperating -- but bring it to us and we'll send you back an answer.'"